Ammonoid biostratigraphy and paleobiogeography of Lower Jurassic sections from Sonora (northwest Mexico) and their tectonic implications

Abstract

The Antimonio terrane is a tectonostratigraphic unit known from the northwestern part of Sonora state of Mexico, where it is developed on the Proterozoic to Permian basement of the Caborca block. The Upper Permian to Lower Jurassic El Antimonio Group, which includes the Lower Jurassic Sierra de Santa Rosa Formation, is its most extensive stratigraphic unit. Biostratigraphic dating of the thick Lower Jurassic sedimentary succession is crucial for the reconstruction of the basin evolution and tectonic history. However, the Early Jurassic ammonite faunas have only been partially treated in previous works; therefore a comprehensive paleontological and biostratigraphical study was carried out.Ammonoids were collected from sections exposing the Sierra de Santa Rosa Formation, including Sierra del Álamo (type locality of the El Antimonio Group), Sierra de Santa Rosa, Pozos de Serna, and Sierra la Jojoba. From 455 moderately preserved specimens, a total of 49 taxa were distinguished, 29 of them identified at the species level, which belong to 22 genera.The ammonoid assemblages represent three stages of the Lower Jurassic, from the Upper Hettangian through the Sinemurian to the Lower Pliensbachian. The provincialism of the fauna tests the recently developed North American regional ammonoid zonation: out of eleven relevant zones, eight were recognized and documented in the studied sections. Stratigraphically from oldest to youngest, these include the Upper Hettangian Rursicostatum Zone, the Lower Sinemurian Involutum and Leslei Zones, and the Upper Sinemurian Carinatum Zone that are identified in Sierra del Álamo. Paltechioceras-dominated assemblages of the uppermost Sinemurian Harbledownense Zone also occur, besides Sierra del Álamo, in the sections at Pozos de Serna and Sierra de Santa Rosa, whereas the lowermost Pliensbachian Imlayi Zone is only represented at Pozos de Serna. The Lower Pliensbachian Whiteavesi and Freboldi Zones occur in sections at Sierra de Santa Rosa and at Sierra la Jojoba. In the latter zone, Fuciniceras perplicatum permits correlation between local sections and represents the first appearance of hildoceratids in North America, only known from the Upper Pliensbachian elsewhere.Although the original proximity of the Antimonio terrane and localities in Nevada is predicted by the Mojave-Sonora megashear hypothesis, paleobiogeographic analyses using multivariate methods revealed only modest similarity of the ammonoid faunas and lend more support for the alternative tectonic model of earlier, pre-Jurassic dextral displacement along the California-Coahuila transform. The early appearance of the hildoceratid Fuciniceras at Sonora and the coeval absence at Nevada also supports our findings. Similarities with coeval faunal from the Insular and Intermontane superterrane of the Canadian Cordillera strengthen arguments for their origin at much lower paleolatitudes and large scale northward displacement as postulated by the Baja BC hypothesis.